Mass spectrometry is an important tool in the analysis of a wide range of chemical compounds. Specifically, mass spectrometers can be used to determine the molecular weight of sample compounds. The analysis of samples by mass spectrometry consists of three main steps—formation of gas phase ions from sample material, mass analysis of the ions to separate the ions from one another according to ion mass, and detection of the ions. A variety of means exist in the field of mass spectrometry to perform each of these three functions. The particular combination of means used in a given spectrometer determine the characteristics of that spectrometer.
To mass analyze ions, for example, one might use a magnetic (B) or electrostatic (E) analyzer. Ions passing through a magnetic or electrostatic field will follow a curved path. In a magnetic field the curvature of the path will be indicative of the momentum-to-charge ratio of the ion. In an electrostatic field, the curvature of the path will be indicative of the energy-to-charge ratio of the ion. If magnetic and electrostatic analyzers are used consecutively, then both the momentum-to-charge and energy-to-charge ratios of the ions will be known and the mass of the ion will thereby be determined. Other mass analyzers are the quadrupole (Q), the ion cyclotron resonance (ICR), the time-of-flight (TOF), and the quadrupole ion trap analyzers.
Before mass analysis can begin, however, gas phase ions must be formed from sample material. If the sample material is sufficiently volatile, ions may be formed by electron ionization (EI) or chemical ionization (CI) of the gas phase sample molecules. For solid samples (e.g. semiconductors, or crystallized materials), ions can be formed by desorption and ionization of sample molecules by bombardment with high energy particles. Secondary ion mass spectrometry (SIMS), for example, uses keV ions to desorb and ionize sample material. In the SIMS process a large amount of energy is deposited in the analyte molecules. As a result, fragile molecules will be fragmented. This fragmentation is undesirable in that information regarding the original composition of the sample—e.g. the molecular weight of sample molecules—will be lost.
For more labile, fragile molecules, other ionization methods now exist. The plasma desorption (PD) technique was introduced by Macfarlane et al. in 1974 (Macfarlane, R. D.; Skowronski, R. P.; Torgerson, D. F., Biochem. Biophys. Res Commoun. 60 (1974) 616). Macfarlane et al. discovered that the impact of high energy (MeV) ions on a surface, like SIMS would cause desorption and ionization of small analyte molecules, however, unlike SIMS, the PD process results also in the desorption of larger, more labile species—e.g. insulin and other protein molecules.
Lasers have been used in a similar manner to induce desorption of biological or other labile molecules. See, for example, VanBreeman, R. B.: Snow, M.: Cotter, R. J., Int. J. Mass Spectrom. Ion Phys. 49 (1983) 35; Tabet, J. C.; Cotter, R. J., Anal. Chem. 56 (1984) 1662; or Olthoff, J. K.; Lys, I.: Demirev, P.: Cotter, R. J., Anal. Instrument. 16 (1987) 93. Cotter et al. modified a CVC 2000 time-of-flight mass spectrometer for infrared laser desorption of involatile biomolecules, using a Tachisto (Needham, Mass.) model 215G pulsed carbon dioxide laser. The plasma or laser desorption and ionization of labile molecules relies on the deposition of little or no energy in the analyte molecules of interest. The use of lasers to desorb and ionize labile molecules intact was enhanced by the introduction of matrix assisted laser desorption ionization (MALDI) (Tanaka, K.; Waki, H.; Ido, Y.; Akita, S.; Yoshida, Y.; Yoshica, T., Rapid Commun. Mass Spectrom. 2 (1988) 151 and Karas, M.; Hillenkamp, F., Anal. Chem. 60 (1988) 2299). In the MALDI process, analyte is dissolve in a solid, organic matrix. Laser light of a wavelength that is absorbed by the solid matrix but not by analyte is used to excite the sample. The matrix is excited directly by the laser. The excited matrix sublimes into the gas phase carrying with it the analyte molecules. The analyte molecules are ionized by proton, electron, or cation transfer from the matrix molecules to the analyte molecules. MALDI is typically used in conjunction with time-of-flight mass spectrometry (TOFMS) and can be used to measure the molecular weights of proteins in excess of 100,000 daltons.
Atmospheric pressure ionization (API) includes a number of methods. Typically, analyte ions are produced from liquid solution at atmospheric pressure. One of the more widely used methods, known as electrospray ionization (ESI), was first suggested by Dole et al. (M. Dole, L. L. Mack, R. L. Hines, R. C. Mobley, L. D. Ferguson, M. B. Alice, J. Chem. Phys. 49, 2240, 1968). In the electrospray technique, analyte is dissolved in a liquid solution and sprayed from a needle. The spray is induced by the application of a potential difference between the needle and a counter electrode. The spray results in the formation of fine, charged droplets of solution containing analyte molecules. In the gas phase, the solvent evaporates leaving behind charged, gas phase, analyte ions. Very large ions can be formed in this way. Ions as large as 1 MDa have been detected by ESI in conjunction with mass spectrometry (ESMS).
ESMS was introduced by Yamashita and Fenn (M. Yamashita and J. B. Fenn, J. Phys. Chem. 88, 4671, 1984). To establish this combination of ESI and MS, ions had to be formed at atmospheric pressure, and then introduced into the vacuum system of a mass analyzer via a differentially pumped interface. The combination of ESI and MS afforded scientists the opportunity to mass analyze a wide range of samples. ESMS is now widely used primarily in the analysis of biomolecules (e.g. proteins) and complex organic molecules.
In the intervening years a number of means and methods useful to ESMS and API-MS have been developed. Much work has been centered on sprayers and ionization chambers. In addition to the original electrospray technique, pneumatic assisted electrospray, dual electrospray, and nano electrospray are now also widely available. Pneumatic assisted electrospray (A. P. Bruins, T. R. Covey, and J. D. Henion, Anal. Chem. 59, 2642, 1987) uses nebulizing gas flowing past the tip of the spray needle to assist in the formation of droplets. The nebulization gas assists in the formation of the spray and thereby makes the operation of the ESI easier. Nano electrospray (M. S. Wilm, M. Mann, Int. J. Mass Spectrom. Ion Processes 136, 167, 1994) employs a much smaller diameter needle than the original electrospray. As a result the flow rate of sample to the tip is lower and the droplets in the spray are finer. However, the ion signal provided by nano electrospray in conjunction with MS is essentially the same as with the original electrospray. Nano electrospray is therefore much more sensitive with respect to the amount of material necessary to perform a given analysis.
Many other ion production methods might be used at atmospheric or elevated pressure. For example, MALDI has recently been adapted by Victor Laid and Alma Burlingame to work at atmospheric pressure (Atmospheric Pressure Matrix Assisted Laser Desorption Ionization, poster #1121, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25–29, 1998) and by Standing et al. at elevated pressures (Time of Flight Mass Spectrometry of Biomolecules with Orthogonal Injection+Collisional Cooling, poster #1272, 4th International Symposium on Mass Spectrometry in the Health and Life Sciences, San Francisco, Aug. 25–29, 1998; and Orthogonal Injection TOFMS Anal. Chem. 71(13), 452A(1999)). The benefit of adapting ion sources in this manner is that the ion optics and mass spectral results are largely independent of the ion production method used.
An elevated pressure ion source always has an ion production region—wherein ions are produced—and an ion transfer region—wherein ions are transferred through differential pumping stages and into the mass analyzer. The ion production region is at an elevated pressure—most often atmospheric pressure—with respect to the analyzer. The ion production region will often include an ionization chamber. In an ESI source, for example, liquid samples are “sprayed” in the “spray chamber” to form ions.
The design of the ionization chamber used in conjunction with API-MS has had a significant impact on the availability and use of these ionization methods with MS. Prior art ionization chambers are inflexible to the extent that a given ionization chamber can be used readily with only a single ionization method and a fixed configuration of sprayers. For example, in order to change from a simple electrospray method to a nano electrospray method of ionization, one had to remove the electrospray ionization chamber from the source and replace it with a nano electrospray chamber (see also, Gourley et al. U.S. Pat. No. 5,753,910, entitled “Angled Chamber Seal for Atmospheric Pressure Ionization Mass Spectrometry”). The ion transfer region will generally include a multipole RF ion guide. Ion guides similar to that of Whitehouse et al. (U.S. Pat. No. 5,652,427) have been shown to be effective in cooling ions and in transferring them from one pressure region to another in a differentially pumped system. In the source of Whitehouse et al., ions are produced by ESI or APCI at substantially atmospheric pressure. These ions are transferred from atmospheric pressure to a first differential pumping region by the gas flow through a glass capillary. Ions are transferred from this first pumping region to a second pumping region by gas flow through a “skimmer”. A multipole in the second differentially pumped region accepts the ions and guides them through a restriction leading through a restriction and into a third differentially pumped region. Meanwhile, collisions with gas flowing through the multipole “cools” the ions resulting in both more efficient ion transfer and the formation of a cool ion beam—which is more readily mass analyzed.
In the scheme of Whitehouse et al. an RF only potential is applied to the multipole. As a result, the multipole is not “selective” but rather transmits ions over a broad range of mass-to-charge (m/z) ratios. Such a range as provided by a prior art multipole is adequate for many applications, however, for some applications—particularly with MALDI—the ions produced may be well out of this range. High m/z ions such as are often produced by the MALDI ionization method are often out of the range of transmission of prior art multipoles.
In other schemes a multipole might be used to guide ions of a selected m/z through the transfer region. Morris et al. (H. R. Morris, et al., High Sensitivity Collisionally-activated Decomposition Tandem Mass Spectrometry on a Novel Quadrupole/Orthogonal-acceleration Time-of-Flight Mass Spectrometer, Rapid Commun. Mass Spectrom. 10, 889 (1996)) use a series of multipoles in their design. One of these is a quadrupole. The quadrupole can be run in a “wide bandpass” mode or a “narrow bandpass” mode. In the wide bandpass mode, an RF-only potential is applied to the quadrupole and ions of a relatively broad range of m/z values are transmitted. In narrow bandpass mode both RF and DC potentials are applied to the quadrupole such that ions of only a narrow range of m/z values are selected for transmission through the quadrupole. In subsequent multipoles, the selected ions may be activated towards dissociation. In this way the instrument of H. R. Morris et al. is able to perform MS/MS with the first mass analysis and subsequent fragmentation occurring in what would otherwise be simply a set of multipole ion guides.
However, this prior art design of Morris et al., when used in “wide bandpass” mode is unable to transmit as wide an m/z range as that of Whitehouse et al. above and certainly not as high an m/z ions as produced by MALDI. The Whitehouse et al. design uses a hexapole. Other prior art designs use an octapole or even a pentapole as the ion guide. Hexapoles, octapoles, and pentapoles are not as good as the Morris design for m/z selection. However, the quadrupole—used in the Morris design—cannot transmit as wide an m/z range as a hexapole, octapole, or pentapole. While some prior art multipoles might be better for transmitting ions of a broad m/z range and others might be better for ion selection, none can transmit high m/z ions such as produced in MALDI (m/z<˜105 Th).